Leaves are known to possess circadian rhythms, but the new study is the first to demonstrate them in whole trees. Researchers looked at the Tasmanian blue gum tree, and found it appears to use its internal clock to regulate its intake of water. These cycles could affect models of climate change, scientists said.

"It had never been shown that the circadian rhythm of the leaf affected the whole tree," said study researcher Ruben Diaz Sierra, a physicist at the National University ofDistance Education in Spain.

"If it works for the tree, it works for the whole forest," he said. Diaz Sierra's colleagues monitored trees in special whole-tree chambers as part of the Hawkesbury Forest Experiment near Sydney, Australia - a broader experiment to study how Australia's eucalypt forests will respond to changes in atmospheric carbon dioxide and climate, a daily reported.

These chambers enable researchers to control the air temperature, humidity and amount of light the trees are exposed to, although these parameters were allowed to vary in this study.

The researchers measured how much water vapour they lost through small openings in the leaves called stomata. The scientists compared these values during overcast nights, when the environment stays mostly constant throughout the evening, with nights that saw dramatic changes in temperature and humidity.

Water loss declined in the six hours after dusk, but increased noticeably during the six hours before dawn, even on nights when temperature and humidity remained constant. Because the environment wasn't changing, the increase can only be explained by the biological clock, said study researcher Victor Resco de Dios of the University of Western Sydney Australia.

"Right now, the models don't take into account the time of the day," Diaz Sierra said, adding that if the time of day affects trees' carbon-dioxide consumption, it would alter models of how climate change will affect ecosystems.